Designable Porous Surfaces for Waterborne Contaminant Capture: A Critical Review of Metal–Organic Frameworks across Radionuclide, Dye and PFAS Domains

Abstract

In just over a decade metal–organic frameworks (MOFs) have gone from being a crystallographic curiosity to one of the most discussed adsorbent families in water treatment, mainly because of their geometry: a MOF is built from metal nodes linked by organic linkers, so the size of its pores, its internal surface area and the chemistry lining its channels can all be tuned almost independently. This review looks at the MOF as a tunable adsorbent surface and questions what that tunability really buys when the goal is a dissolved contaminant. We focus on three classes where MOFs have published their most convincing results, radionuclides, synthetic dyes and per- and poly-fluoroalkyl substances (PFAS), and gather reported Langmuir capacities together with the isotherm, kinetic and mechanistic evidence behind them. The numbers are mind-blowing. Amidoxime- and chitosan-decorated frameworks shuttle uranium(VI) past 2500 mg/g, an ultrasound-assisted ZIF-67 pushes malachite green past 3600 mg/g, and ferrocyanide- and oxalate-trapped frameworks remove caesium and strontium out of solution with real selectivity. Almost all of these systems are described by Langmuir isotherms and pseudo-second-order kinetics with coordination to open metal sites, ion exchange and electrostatic attraction driving the capture and for the fluorinated tails of PFAS hydrophobic association. We devote equal space to the major shortcoming of the field, the variable stability of many frameworks in actual water, as also to cost and regeneration, for these are what still distinguish an elegant isotherm from a working process. Tables and figure captions are provided for immediate use.